Potasium bicarbonate is not easy to obtain, unless you know some whole saler of agricultural products. Potasium bicarbonate is mostly purchased by roots vegetable Bio-culturing farms to control pests.
Potasium bicarbonate works better than Sodium bicarbonate with roses because potasium strengthen the roses, giving longer flowering. Potasium assists roots growth and acts as a feuillage feed. Whereas sodium could dry out the moisture in the feuillage.
It depends on what diseases you are tackling. For roses, there are many ways to deal with their persistance fungal tendency. A good rose grower should learn how to adapt their accessiblity to treatment, to benefit the fulliest of joy.
Personally, I use Sodium bicarbonate, clematis leaf solution or horsetail solution or silver fern solution and Bordelais for fungus control, togather with some other feuillage feed like comfrey or nettles solutions to strengthen my roses. I believe that fungus control spray will injure my rose in some ways, therefore I always make an effort to replemish their lost by some beneficial elements.
There are a lot of beneficial wild herbs around you, and are totally free. If you wish you can do some researches on them like Comfrey, Nettles, and Silver Fern and Horsetail. They are natural feed and pest control that are extremely effective and of no side effects.
Hope it helps.